SX-838 going into protect, some troubleshooting done

rplst8

New Member
Hi all, first post to these forums.

I have an SX-838 I picked up a while ago actually from ebay. When I first got it, it would play for about 30-45 minutes and then go into protect. It would cycle on and off and as time went on the cycles were quicker and quicker. I only paid like $150 for it, so I wasn't expecting much. I ended up just shelving it and figured I'd deal with it later as I got busy with other things.

So fast forward to now, I've gotten interested in this thing again and getting it working. When I plugged it in the other night it started doing the same thing right from the get go. So I grabbed my DMM and a service manual and started poking around.

First I checked the bias and offset. The left channel checks out OK. I tightened them up a little but it was within spec as is. Then I checked the right channel. I'm not sure which is offset and which is bias but the service manual has you check pin 20 to ground on the power amp board and bring that to 0 mV (which I did at first). Then you check between pins 22 and 23 and bring that under 30 mV. So during this time, the amp clicks off again and voltage measured on pin 5 to gnd just goes nuts. It's all over the place. I stuck an old analog meter on it so I could watch it swing, and it was going about +30 to -10 or so.

As time went on, the swings became more frequent and erratic. During this, the left channel was totally stable.

I checked all the power supply rails that I could. The power supply rails marked +45 and -45 were like +43.5 and -43.5. The rail that is labeled 35.5 was like 34.5. The 13V and -13V rails were rock solid at 13V and -13V damn near on the dot.

Basically, all the voltages were very very close to what they should be, and pretty solid. I saw +/- 0.1V fluctuation at times but only on the unregulated supplies. I also checked the supply going to the phono preamp as I've heard that can cause problems if there is a short. Again 22V and 13V were rock solid.

I measured the bias and offset pots too. They seem to be good. As I turned them (with the amp off) I got smooth transitions throughout the rotation of the wiper.

When the amp is not in protect, both channels will pass good sounding audio. Just as it goes into protect, the right side lets out a pretty loud bzzzt, and then the protective circuitry mutes the outputs.

Any ideas where to look next?

Thanks,
RL
 
Yes: The sx-838 has a pair of 2sa726's and a 2sc1451 in each power amplifier channel. AW#H-031 q1,2,3,4 and Q5,6

Both transistor numbers are known troublemakers, especially the 2sc1451.

replace the q1,2,3,4 2sa726's with 512-ksa992FBU
512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

replace the q5,6 2sc1451 with 512-KSC3503DSTU
512-KSC3503dstu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.28 ea

from mouser.com. you might want to look into recapping since you are going through the trouble of pulling the board, and readjusting it after ward. NEW Trimpots VR1,2,3,4 would be a very VERY good precaution. If you're game we'll come up with the parts numbers, if they aren't in here already... :yes:

ask questions if you do not know positively how to install the transistor leads in the correct order - as the replacements may NOT be the same lead layout as the originals.


read this recent post:
http://www.audiokarma.org/forums/showpost.php?p=6299333&postcount=10

Then ask questions and we'll figure out how detailed my instructions need to be.


oh, and welcome to AK !!! later on, go to the introductions forum and introduce yourself !! :D
 
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Yes: The sx-838 has a pair of 2sa726's and a 2sc1451 in each power amplifier channel. AW#H-031 q1,2,3,4 and Q5,6

Both transistor numbers are known troublemakers, especially the 2sc1451.

replace the q1,2,3,4 2sa726's with 512-ksa992FBU
512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

replace the q5,6 2sc1451 with 512-KSC3503DSTU
512-KSC3503dstu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.28 ea

from mouser.com. you might want to look into recapping since you are going through the trouble of pulling the board, and readjusting it after ward. NEW Trimpots VR1,2,3,4 would be a very VERY good precaution. If you're game we'll come up with the parts numbers, if they aren't in here already... :yes:

ask questions if you do not know positively how to install the transistor leads in the correct order - as the replacements may NOT be the same lead layout as the originals.

Then ask questions and we'll figure out how detailed my instructions need to be.

Thanks for the quick answer and warm welcome. Recapping seems like a good idea as you say, because it looks like it's going to be a good bit of work anyway. The trim pots seemed pretty solid. At first I thought the trim pots might have been the issue and I started to look for replacements on Mouser.com. I couldn't find any direct replacements. I'd love some help finding some.

Another question - how do most people go about getting these boards out? Undoing all the wire wrap?
 
Don't undo wire wraps, work slack through cutting tie-wraps and unbundling.

worst case we unsolder the pin while leaving the wire wrap alone. (and document the daylights out of it first).

there are no direct replacements. modern stuff must be chivvied around a bit to get the wires into the holes. we use multi-turn for the dc balance/offset adjusts, and single turn for the idle current adjusts.

I'll get parts numbers later...
 
I have have the same unit - same problem. Mine started acting up - playing normally, then going into protect mode - on and off. No noise from it really - just tripping the protection breaker. I shut it down and shelved it until I could get to it. Please post progress/results! I'm looking to get at mine this winter!
 
I'll get parts numbers later...

Thanks.

I've begun creating a list of things to buy over at Mouser.com, including all the caps for the protection and power supply boards. I found a thread from last year detailing this. I also found replacements for the output drivers (so I threw those in the cart as well for a later date when I inevitably blow something up).

However, I didn't find a complete list of equivalents for the transistors on the power supply and protection boards - just the beginnings of a list. Plus, even some of the parts listed a few years ago (thread was form 2009 or 2010) are now obsoleted.

Right now I have in my cart:
all electrolytic caps for the power amp, power supply, and protection boards.
all transistors for the power amp board (except the 2SB528 and 2SD358, which no one seems to bark loudly about).

No diodes, no resistors, no ceramic or mylar caps, no transistors for other boards. See any need to source any of these too?

I'd also like to replace the mode indicator lights for the SPK-A selector and for the AUX selector, but I haven't begun to research that yet.
 
here's matt's list.

put YOUR list of found replacements up and make it easier on me, I'll vet the list and the parts numbers, it'll be faster than me being able to get to it and start almost from scratch. highlight or separate the "bad" parts numbers....

That's why I always include the board number and component location (xyz-123 q11) with each part, the lists can be scrambled/separated arbitrarily, and readily reassembled.

Mouser.com's always dropping or reassigning detailed parts numbers.

Since a list was asked for, a list you shall get! :D

Pioneer SX-838 recap list

Equalizer Amp Board (AWF-011-0)

AWF-011-0: C1: 1uF 25v CSSA: 647-UKL1H010KDDANA 1uF 50v
AWF-011-0: C2: 1uF 25v CSSA: 647-UKL1H010KDDANA 1uF 50v
AWF-011-0: C7: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWF-011-0: C8: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWF-011-0: C11: 3.3uF 25v CEANL: 647-UKL1H3R3KDDANA 3.3uF 50v
AWF-011-0: C12: 3.3uF 25v CEANL: 647-UKL1H3R3KDDANA 3.3uF 50v
AWF-011-0: C17: 100uF 25v CEA: 647-UPW1E101MED 100uF 25v
AWF-011-0: C18: 220uF 16v CEA: 647-UPW1V221MPD 220uF 35v

Q1: 2SA725: 512-KSA992FBU
Q2: 2SA725: 512-KSA992FBU
Q3: 2SC1313: 512-KSC1845FTA:
Q4: 2SC1313: 512-KSC1845FTA:
Q5: 2SC1313: 512-KSC1845FTA:
Q6: 2SC1313: 512-KSC1845FTA:

Microphone Amp Board (AWM-064-0)

AWM-064-0: C2: 220uF 35v CEA: 647-UPW1V221MPD 220uF 35v
AWM-064-0: C3: .47uF 16v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWM-064-0: C6: 220uF 6.3v CEA: 647-UPW1C221MPD 220uF 16v
AWM-064-0: C7: .47uF 50v CEANL: 667-ECQ-V1H474JL3 .47uF 50v

Q1: 2SC1344: 512-KSC1845FTA:
Q2: 2SC1344: 512-KSC1845FTA:

Control Amp Board (AWG-029-0)

AWG-029-0: C3: 47uF 35v CEA: 647-UPW1V470MED 47uF 35v
AWG-029-0: C4: 47uF 35v CEA: 647-UPW1V470MED 47uF 35v
AWG-029-0: C7: 10uF 25v CSZA: 647-UKL1E100KDDANA 10uF 25v
AWG-029-0: C8: 10uF 25v CSZA: 647-UKL1E100KDDANA 10uF 25v
AWG-029-0: C13: 100uF 10v CEA: 647-UPW1C101MED 100uF 16v
AWG-029-0: C14: 100uF 10v CEA: 647-UPW1C101MED 100uF 16v
AWG-029-0: C15: 4.7uF 25v CEANL: 647-UKL1H4R7KDDANA 4.7uF 50v
AWG-029-0: C16: 4.7uF 25v CEANL: 647-UKL1H4R7KDDANA 4.7uF 50v
AWG-029-0: C29: 10uF 25v CSZA: 647-UKL1E100KDDANA 10uF 25v
AWG-029-0: C30: 10uF 25v CSZA: 647-UKL1E100KDDANA 10uF 25v
AWG-029-0: C31: 10uF 25v CSZA: 647-UKL1E100KDDANA 10uF 25v
AWG-029-0: C32: 10uF 25v CSZA: 647-UKL1E100KDDANA 10uF 25v
AWG-029-0: C35: 100uF 6.3v CEA: 647-UPW1C101MED 100uF 16v
AWG-029-0: C36: 100uF 6.3v CEA: 647-UPW1C101MED 100uF 16v
AWG-029-0: C37: 47uF 35v CEA: 647-UPW1V470MED 47uF 35v
AWG-029-0: C38: 47uF 35v CEA: 647-UPW1V470MED 47uF 35v
AWG-029-0: C39: 2.2uF 25v CSZA: 647-UKL1H2R2KDDANA 2.2uF 50v
AWG-029-0: C40: 2.2uF 25v CSZA: 647-UKL1H2R2KDDANA 2.2uF 50v

Q3: 2SA725: 512-KSA992FBU
Q4: 2SA725: 512-KSA992FBU
Q5: 2SC1312: 512-KSC1845FTA:
Q6: 2SC1312: 512-KSC1845FTA:

Power Amp Board (AWH-031-A) (Q1 & Q3 and Q2 & Q4 should be gain matched)

AWH-031-A: C1: 1uF 25v CSSA: 647-UKL1H010KDDANA 1uF 50v
AWH-031-A: C2: 1uF 25v CSSA: 647-UKL1H010KDDANA 1uF 50v
AWH-031-A: C3: .47uF 25v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWH-031-A: C4: .47uF 25v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWH-031-A: C7: 33uF 16v CEA: 647-UPW1C330MDD 33uF 16v
AWH-031-A: C8: 33uF 16v CEA: 647-UPW1C330MDD 33uF 16v
AWH-031-A: C11: 330uF 10v CEA: 647-UPW1A331MPD 330uF 10v
AWH-031-A: C12: 330uF 10v CEA: 647-UPW1A331MPD 330uF 10v
AWH-031-A: C13: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWH-031-A: C14: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v

Q1: 2SA725: 512-KSA992FBU
Q2: 2SA725: 512-KSA992FBU
Q3: 2SA725: 512-KSA992FBU
Q4: 2SA725: 512-KSA992FBU
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU

Protection Board (AWM-025-C)

AWM-025-C1: .22uF 10v CSSA: 667-ECQ-V1H224JL .22uF 50v
AWM-025-C2: .22uF 10v CSSA: 667-ECQ-V1H224JL .22uF 50v
AWM-025-C3: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWM-025-C4: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWM-025-C5: 4.7uF 25v CEA: 647-UPW1E4R7MDD 4.7uF 25v
AWM-025-C6: 100uF 16v CEA: 647-UPW1E101MED 100uF 25v

Q6: 2SC945: 512-KSD1616ALBU
Q7: 2SC1384: 512-KSC2690AYS
1N4004 flyback diode is added between pins 9 (anode) and 10 (cathode)

Power Supply Board (AWR-055-0)

AWR-055-0: C8: 470uF 80v CEA: 647-UPW2A471MHD 470uF 100v
AWR-055-0: C9: 470uF 80v CEA: 647-UPW2A471MHD 470uF 100v
AWR-055-0: C10: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWR-055-0: C11: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWR-055-0: C12: 220uF 16v CEA: 647-UPW1V221MPD 220uF 35v
AWR-055-0: C13: 2200uF 10v CEA: 647-UPW1C222MHD 2200uF 16v
AWR-055-0: C14: 220uF 50v CEA: 647-UPW1H221MPD 220uF 50v
AWR-055-0: C15: 470uF 16v CEA: 647-UPW1E471MPD6 470uF 25v

BOM List:

647-UKL1H010KDDANA | 4
647-UPW1A331MPD | 6
647-UKL1H3R3KDDANA | 2
647-UPW1E101MED | 2
647-UPW1V221MPD | 3
667-ECQ-V1H474JL3 | 4
647-UPW1C221MPD | 1
647-UKL1H2R2KDDANA | 2
647-UKL1H4R7KDDANA | 2
647-UPW1V470MED | 4
647-UPW1J101MPD | 4
667-ECQ-V1H224JL | 2
647-UPW1E4R7MDD | 1
647-UPW1H221MPD | 1
647-UPW1C101MED | 4
647-UPW1E471MPD6 | 1
647-UPW1C330MDD | 2
647-UKL1E100KDDANA | 6
647-UPW2A471MHD | 2
647-UPW1C222MHD | 1
512-KSC2690AYS | 1
512-KSD1616ALBU | 1
512-KSC3503DSTU | 2
512-KSA992FBU | 8
512-KSC1845FTA | 8

All parts were confirmed to be in stock at the time of this post.

.
 
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here's matt's list.

put YOUR list of found replacements up and make it easier on me, I'll vet the list and the parts numbers, it'll be faster than me being able to get to it and start almost from scratch. highlight or separate the "bad" parts numbers....

That's why I always include the board number and component location (xyz-123 q11) with each part, the lists can be scrambled/separated arbitrarily, and readily reassembled.

Mouser.com's always dropping or reassigning detailed parts numbers.

Colors/emphasis:
no-highlighting - Matches your suggested part
Italics - My selection from another SX-838 BOM, needs vetted
Red - Seemingly incorrect part I got from another thread here on AK
Orange - Possibly correct but unavailable
Green - Replacement for orange that I sourced, needs vetted
Bold - My selection from another SX-838 BOM, seems acceptable and is available, but a difference from the BOM you posted. Needs vetted.

I've only listed the boards that I'm working on, power amp, protection, and power supply. Here's what I have so far from Mouser as of today:

Power Amp Board (AWH-031-A)

AWH-031-A: C1: 1uF 25v CSSA: 667-ECQ-V1H105JL3 1uF 50v (seems acceptable but differs from your list)
AWH-031-A: C2: 1uF 25v CSSA: 667-ECQ-V1H105JL3 1uF 50v (seems acceptable but differs from your list)
AWH-031-A: C3: .47uF 25v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWH-031-A: C4: .47uF 25v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWH-031-A: C7: 33uF 16v CEA: 647-UPW1C330MDD 33uF 16v
AWH-031-A: C8: 33uF 16v CEA: 647-UPW1C330MDD 33uF 16v
AWH-031-A: C11: 330uF 10v CEA: 647-UPW1A331MPD 330uF 10v
AWH-031-A: C12: 330uF 10v CEA: 647-UPW1A331MPD 330uF 10v
AWH-031-A: C13: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWH-031-A: C14: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v

Q1: 2SA725: 512-KSA992FBU
Q2: 2SA725: 512-KSA992FBU
Q3: 2SA725: 512-KSA992FBU
Q4: 2SA725: 512-KSA992FBU
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU
Q7: 2SD358: 512-KSC2690AYSTU
Q8: 2SD358: 512-KSC2690AYSTU
Q9: 2SB528: 512-KSA940TU
Q10: 2SB528: 512-KSA940TU
Q11: 2SC1079: 863-MJ21194G
Q12: 2SC1079: 863-MJ21194G
Q13: 2SA679: 863-MJ21193G
Q14: 2SA679: 863-MJ21193G

Protection Board (AWM-025-C)

AWM-025-C1: .22uF 10v CSSA: 667-ECQ-V1H224JL .22uF 50v 667-ECQ-V1H474JL3 .47uf 50v (corrected)
AWM-025-C2: .22uF 10v CSSA: 667-ECQ-V1H224JL .22uF 50v 667-ECQ-V1H474JL3 .47uf 50v (corrected)
AWM-025-C3: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWM-025-C4: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWM-025-C5: 4.7uF 25v CEA: 647-UPW1E4R7MDD 4.7uF 25v
AWM-025-C6: 100uF 16v CEA: 647-UPW1E101MED 100uF 25v

Q1: 2SC869: 512-KSC2310YBU (nothing available in KSC2310 either)
Q2: 2SC869: 512-KSC2310YBU (nothing available in KSC2310 either)
Q3: 2SC945: 512-KSC945CLTA 512-KSC945YTA (found another)
Q4: 2SC945: 512-KSC945CLTA 512-KSC945YTA (found another)
Q5: 2SA733: 512-KSA733CLTA (end-of-life, but available - many other KSA733 parts available)
Q6: 2SC945: 512-KSD1616ALBU (I had 512-KSD1616ALTA, doesn't seem to matter)
Q7: 2SC1384: 512-KSC2690AYS 512-KSC2383YBU (no mention of diode with 512-KSC2383YBU)
1N4004 flyback diode is added between pins 9 (anode) and 10 (cathode)

Power Supply Board (AWR-055-0)

AWR-055-0: C8: 470uF 80v CEA: 647-UPW2A471MHD 470uF 100v 647-UPS2A471MHD 470uF 100v
AWR-055-0: C9: 470uF 80v CEA: 647-UPW2A471MHD 470uF 100v 647-UPS2A471MHD 470uF 100v
AWR-055-0: C10: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWR-055-0: C11: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWR-055-0: C12: 220uF 16v CEA: 647-UPW1V221MPD 220uF 35v 647-UPW1E221MPD 220uF 25v
AWR-055-0: C13: 2200uF 10v CEA: 647-UPW1C222MHD 2200uF 16v
AWR-055-0: C14: 220uF 50v CEA: 647-UPW1H221MPD 220uF 50v 647-UPW1J221MPD6 220uF 63v
AWR-055-0: C15: 470uF 16v CEA: 647-UPW1E471MPD6 470uF 25v

Q1: 2SD313-E: 512-KSC2073TU
Q2: 2SD313P E or D: 512-KSC2073TU
Q3: 2SC945-Q or R: 512-KSC945LBU (end-of-life, but available - many other KSC945 parts available)
 
647-UHE2A471MHD 470uf 100v $1.54 10,000 hrs preferred
647-UPS2A471MHD 470uf 100v $1.53 3,000 hrs
-------
other caps are fine
-------
awm025 q3,q4,q5 leave them original
-------
512-1n4004
-------
awm025 Q7: 2SC1384: 512-KSC2690AYS use this, not the c2383...
-------
awh031 Q7: 2SD358: 512-KSC2073TU
awh031 Q8: 2SD358: 512-KSC2073TU
-------
AWR055 Q3: 2SC945 512-KSC2383YTA
-------
STILL IN PROGRESS!!!
 
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647-UHE2A471MHD 470uf 100v $1.54 10,000 hrs preferred
647-UPS2A471MHD 470uf 100v $1.53 3,000 hrs
-------
other caps are fine
-------
awm025 q3,q4,q5 leave them original
-------
512-1n4004
-------
awm025 Q7: 2SC1384: 512-KSC2690AYS use this, not the c2383...
-------
awh031 Q7: 2SD358: 512-KSC2073TU
awh031 Q8: 2SD358: 512-KSC2073TU
-------
AWR055 Q3: 2SC945 512-KSC2383YTA
-------
STILL IN PROGRESS!!!

Do I have enough info here to at least order the parts for the power amp board?
 
The parts won't go out for shipping at least until monday, so better to wait for Mark. Considering Christmas is Tuesday, Mouser might be closed on Monday (maybe). So I'd wait for Mark, and get the list all at one time. (We all get impatient on our 1st piece of gear, or 20th).
 
Larry's assessment of the timing is pretty good, probably about boxing day I'll be onto and back into the lists, reread this thread and see what's missing.

or you could modify Mattsd's list with the steps i posted and repost the result, save me a bit of time to see what you have down ready (as it were) to go instead of me doing it and posting it.

yep, I'm pretty jammed up due to the season - you wanna talk FRUSTRATED?? :D

I have something big and expensive and electronic that just came in the door as MINE and expressly not working, and it just has to SIT there, because I don't even have the time to check out it's problems, and I might not be able to get to it until late spring or early summer... man was I SURPRISED when I WON THAT auction... the first words out of my mouth was an involuntary "oh shit!!! i WON??? "... I was the little dog that caught the locomotive.... and NOW WHAT was I going to DO??? :yikes:

I haven't even had supper yet tonight....
 
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or you could modify Mattsd's list with the steps i posted and repost the result, save me a bit of time to see what you have down ready (as it were) to go instead of me doing it and posting it.

OK, updated with your changes. Note, I ordered from Mouser a few days ago for the power amp board (AWH-031-A) as listed below with your suggested changes. I'll make another post about that after this update.

Colors/emphasis:
Blue - Updated per your instructions
no-highlighting - Matches your suggested part - or now vetted by you
Italics - My selection from another SX-838 BOM, needs vetted
Orange - Possibly correct but unavailable
Bold - My selection from another SX-838 BOM, seems acceptable and is available, but a difference from the BOM you posted. Now vetted. In some cases just an alternate part.


I've only listed the boards that I'm working on, power amp, protection, and power supply. Here's what I have so far from Mouser as of today:

Power Amp Board (AWH-031-A)

AWH-031-A: C1: 1uF 25v CSSA: 667-ECQ-V1H105JL3 1uF 50v
AWH-031-A: C2: 1uF 25v CSSA: 667-ECQ-V1H105JL3 1uF 50v
AWH-031-A: C3: .47uF 25v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWH-031-A: C4: .47uF 25v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWH-031-A: C7: 33uF 16v CEA: 647-UPW1C330MDD 33uF 16v
AWH-031-A: C8: 33uF 16v CEA: 647-UPW1C330MDD 33uF 16v
AWH-031-A: C11: 330uF 10v CEA: 647-UPW1A331MPD 330uF 10v
AWH-031-A: C12: 330uF 10v CEA: 647-UPW1A331MPD 330uF 10v
AWH-031-A: C13: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWH-031-A: C14: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v

Q1: 2SA726: 512-KSA992FBU
Q2: 2SA726: 512-KSA992FBU
Q3: 2SA726: 512-KSA992FBU
Q4: 2SA726: 512-KSA992FBU
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU
Q7: 2SD358: 512-KSC2073TU
Q8: 2SD358: 512-KSC2073TU
Q9: 2SB528: 512-KSA940TU
Q10: 2SB528: 512-KSA940TU
Q11: 2SC1079: 863-MJ21194G
Q12: 2SC1079: 863-MJ21194G
Q13: 2SA679: 863-MJ21193G
Q14: 2SA679: 863-MJ21193G

Protection Board (AWM-025-C)

AWM-025-C1: .22uF 10v CSSA: 667-ECQ-V1H224JL .22uF 50v
AWM-025-C2: .22uF 10v CSSA: 667-ECQ-V1H224JL .22uF 50v
AWM-025-C3: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWM-025-C4: 330uF 6.3v CEA: 647-UPW1A331MPD 330uF 10v
AWM-025-C5: 4.7uF 25v CEA: 647-UPW1E4R7MDD 4.7uF 25v
AWM-025-C6: 100uF 16v CEA: 647-UPW1E101MED 100uF 25v

Q1: 2SC869: 512-KSC2310YBU (nothing available in KSC2310 either)
Q2: 2SC869: 512-KSC2310YBU (nothing available in KSC2310 either)

Q3: 2SC945: leave original
Q4: 2SC945: leave original
Q5: 2SA733: leave original

Q6: 2SC945: 512-KSD1616ALBU (I had 512-KSD1616ALTA, doesn't seem to matter)
Q7: 2SC1384: 512-KSC2690AYS
1N4004 flyback diode is added between pins 9 (anode) and 10 (cathode) 512-1n4004

Power Supply Board (AWR-055-0)

AWR-055-0: C8: 470uF 80v CEA: 647-UHE2A471MHD 470uF 100v
AWR-055-0: C9: 470uF 80v CEA: 647-UHE2A471MHD 470uF 100v
AWR-055-0: C10: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWR-055-0: C11: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v
AWR-055-0: C12: 220uF 16v CEA: 647-UPW1V221MPD 220uF 35v 647-UPW1E221MPD 220uF 25v
AWR-055-0: C13: 2200uF 10v CEA: 647-UPW1C222MHD 2200uF 16v
AWR-055-0: C14: 220uF 50v CEA: 647-UPW1H221MPD 220uF 50v 647-UPW1J221MPD6 220uF 63v
AWR-055-0: C15: 470uF 16v CEA: 647-UPW1E471MPD6 470uF 25v

Q1: 2SD313-E: 512-KSC2073TU
Q2: 2SD313P E or D: 512-KSC2073TU
Q3: 2SC945-Q or R: 512-KSC2383YTA (512-KSC945LBU end-of-life, but available - many other KSC945 parts available)
 
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OK, updated with your changes. Note, I ordered from Mouser a few days ago for the power amp board (AWH-031-A) as listed below with your suggested changes. I'll make another post about that after this update.

OK, so I ordered some stuff from Mouser from the last list above for the AWH-031 board only (power amp). Since that seemed pretty straightforward, the list of the problem parts was mostly vetted, and heck - that *was* the original problem I was having.

So the first thing I did was replace the "pesky" and often complained about 2SC1451s. I didn't bother doing anything else as I just wanted to get my feet wet, and seeing as those are very often the problem I went after those.

I reassembled most the board and heat sink so I could test. First thing I did was check the offset and bias since those were what gave me obvious, measurable problems before. Good news, I could now get very stable readings on my DMM and I was able to adjust the bias with no issue, other than running out of adjustment on the one pot - no matter it was close enough for horseshoes. Probably a sign of a cap starting to go south. It was within a millivolt or two and the manual says under 30 mV so I called it done.

I hooked up a speaker and got great sound from the left channel, which worked originally. I should also mention that the protection circuit never kicked in and I had it powered up for over an hour (prior to this it would cycle into protect after 10-20 minutes, cycling faster as time went by).

I hooked the speaker to the right channel and got sound, but it had a lot of distortion. I'm wondering if one of the output drivers was damaged by the bad 1451 (or something I did). I suppose the 2SD358s, 2SB528s, or 2SA726s could have been damaged by the 1451 or by me. I dunno.

I have replacements here for all of them. I ran out of time tonight, but I will probably use my scope to find the source. Any thoughts? I do know the problem is not before the power amp board. I tested with input at the power-amp inputs on the back and with aux and tape input. I'm 99.9% certain the distortion is happening on the power amp board.

Note: mattsd seems to have made a transcription error (which I have repeated until now, and corrected below). AWH-031 Q1-Q4 should be 2SA726 not 2SA725. Not sure if that changes the replacement part selection.

Power Amp Board (AWH-031-A) (transistors for reference)

Q1: 2SA726: 512-KSA992FBU
Q2: 2SA726: 512-KSA992FBU
Q3: 2SA726: 512-KSA992FBU
Q4: 2SA726: 512-KSA992FBU
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU
Q7: 2SD358: 512-KSC2073TU
Q8: 2SD358: 512-KSC2073TU
Q9: 2SB528: 512-KSA940TU
Q10: 2SB528: 512-KSA940TU
Q11: 2SC1079: 863-MJ21194G
Q12: 2SC1079: 863-MJ21194G
Q13: 2SA679: 863-MJ21193G
Q14: 2SA679: 863-MJ21193G
 
a725 (Vceo 30v), a726 (Vceo 50v) both a992's (Vceo 120v :D )

yes, dying c1451 could hurt driver or outputs (probably NOT you...). Look for other than 0.6v emitter to base, on drivers (q7-10) or outputs (q11-14).
replace in pairs, and check emitter resistors(0.5 ohm and 150 ohm) and base drive (4.7 ohms) carefully - all with one leg lifted.
 
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yes, dying c1451 could hurt driver or outputs (probably NOT you...). Look for other than 0.6v emitter to base ... outputs (q11-14).


Turns out my Q11-Q14 are SB539 and SD287 pairs.

The left channel ones meter at ~506 and 520 mV respective, and the right side (side with distortion) meter ~495 and 521 mV respective.

Too low?
 
Update:

I found a burnt trace. Jumped it with a wire, now back to square one. Powers up, but cycles into protect. after 10 min warm up, then faster.

I'm going after the 2SA726s next.
 
replace the diode (D4/5) across the idle current pot.

when doing this you MUST start back from zero idle current - with the way things behaved it was probably pulling down a higher idle current setting.

It probably was damaged when the 2sc1451 let go, and it "saved" the pot, drivers and outputs, at the cost of it shorting.

mouser 512-1n4148 (1n914 etc Radio Shack Model: 1N914 | Catalog #: 276-1122 1N914/4148-Type Diode $1.99 save on shipping...)

Or you could temporarily swap d4 and d5 and see if the distortion and idle current difficulties moves to the other channel - UNLESS you happen to have a diode available. Since it IS a installed in the circuit across the idle current pot as a safety measure, a 1n4004 could suffice for now, the signal diode is preferred.
 
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Update:

Before I saw your reply, I finished replacing the 2SA726s and all the specified caps on the right channel only. Reassembled...

TADA! It's alive! So far so good at least.

So far, replaced:

Power Amp Board (AWH-031-A)

AWH-031-A: C2: 1uF 25v CSSA: 667-ECQ-V1H105JL3 1uF 50v
AWH-031-A: C4: .47uF 25v CSSA: 667-ECQ-V1H474JL3 .47uF 50v
AWH-031-A: C8: 33uF 16v CEA: 647-UPW1C330MDD 33uF 16v
AWH-031-A: C12: 330uF 10v CEA: 647-UPW1A331MPD 330uF 10v
AWH-031-A: C14: 100uF 50v CEA: 647-UPW1J101MPD 100uF 63v

Q2: 2SA726: 512-KSA992FBU
Q4: 2SA726: 512-KSA992FBU
Q5: 2SC1451: 512-KSC3503DSTU*
Q6: 2SC1451: 512-KSC3503DSTU

* Left channel

I'm not sure if the 1451s were the problem to begin with. I metered them and got ~0.6V of drop base to emitter and base to collector, OL in all other directions/combinations. Are there some other tests I can do to verify they are bad? Can a transistor go bad and still meter with a diode checker correctly? The unit was intermittent... maybe the 1451s and/or 726s were just flaky? If there was still some other fault in the circuit, the new components wouldn't make the circuit more tolerant would it?

Other than replacing the 1451, I haven't done anything else to the left channel as it's been working fine ever since I got the unit 7-8 years ago.

Should I bother replacing the rest on the left channel? Should I also replace those diodes as you suggested? Oh, and for the record, the trace that burned out was between Q10 collector and C20.
 
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